Ohmic contact to Gallium Arsenide using epitaxially deposited Cobalt Digermanide

ABSTRACT

A partially ionized beam (PIB) deposition technique is used to  heteroepitally deposit a thin film of CoGe 2  (001) on GaAs (100) substrates 14. The resulting epitaxial arrangement is CoGe 2  (001) GaAs (100). The best epitaxial layer is obtained with an ion energy 1100 eV to 1200 eV and with a substrate temperature of approximately 280° Centigrade. The substrate wafers are treated only by immersion in HF:H 2  O 1:10 immediately prior to deposition of the epitaxial layer. Contacts grown at these optimal conditions display ohmic behavior, while contacts grown at higher or lower substrate temperatures exhibit rectifying behavior. Epitaxial formation of a high melting point, low resistivity cobalt germanide phase results in the formation of a stable contact to n-GaAs.

UNITED STATES GOVERNMENT INTEREST

The invention described herein may be manufactured, used and licensed byor for the United States Government for United States Governmentpurposes.

This application claims benefit of Provision Appln. 60/032,655 filedDec. 9, 1996 and 60/022,541 filed Jul. 22, 1996.

TECHNICAL FIELD

The present invention relates in general to low-temperature epitaxialgrowth of thin films. In particular, the present invention relates tothe use of a partially ionized beam deposition of CoGe₂ (001) thinfilms, i.e. Cobalt Digermanide on GaAs (100) substrates i.e. GallionArsenide.

BACKGROUND ART

Contacts to n-GaAs doped-substrates resulting in either Ohmic orrectifying behavior are technologically important for use in electronicand optoelectronic devices and circuits. Ideally, an Ohmic contactshould allow the required current with a voltage drop that issubstantially small compared to the drop across the active region of thedevice, so as not to significantly disturb device operation. Duringfabrication of GaAs devices, annealing temperatures routinely reachabout 400° C., and can be as high as 600-800° C. during solar cell andself-aligning device processing. A key issue in manufacturing suchdevices is producing thermally stable and reliable contacts.

For manufacturing contacts to n-GaAs doped substrates, no elementalmetal offers a low Schottky barrier which would avoid undesirablerectifying behavior. The most commonly used Ohmic contact to n-GaAsmaterial are systems based on the Au-Ge eutectic. When an Au-Ge film onGaAs doped material is heated to the eutectic temperature (356°), anOhmic contact is formed. Au-Ge, however, does not easily wet GaAs. Topromote wetting, small amounts of Ni (from 2-11 wt %) are added duringdeposition. Many other schemes of Ohmic contact formation have beenconceived. However, most require metallurgical interaction with the GaAs(alloying) induced by a high temperature (often >400° C.) thermalanneal, a time-consuming process often resulting in undesirable contactmorphology.

Other techniques, have been attempted without particular success. Forexample, sequential sputtering of Co and Ge followed by an anneal toproduce Ge-rich Co-Ge contacts has been carried-out. However, the annealstep was shown to induce chemical interaction with the GaAs, and in mostcases produces rectifying behavior for the contacts. The resultingproduct is entirely unsuitable as a contact in a semiconductor device.

In summary, the conventional art, in particular that using Au-Geeutectic to make ohmic contact to GaAs, suffers from lack of temperaturestability resulting in a lack of film/contact uniformity. The alloyingprocess produces a non-uniform contact with the substrate. Further,undesirable interactions with the substrate occur, as well as otherdrawbacks illustrated.

SUMMARY OF THE INVENTION

One object of the present invention is the provision of uniform, stableOhmic contacts to GaAs doped substrates, not currently available withthe current conventional art.

Another object of the present invention is to provide a less expensiveOhmic contact.

A further object of the present invention is to provide an Ohmic contactthat is able to endure high temperature post-processing due to inherentthermal stability of the contact.

Yet a further object of the present invention is to provide a stableohmic contact that is useful in high-power devices.

Still an additional object of the present invention is to provide a lowresistivity Ohmic contact to GaAs doped substrates.

These and other advantages of the present invention are realized througha process for forming a contact on a GaAs doped substrate withoutalloying. During the process a partially ionized beam is generated toinclude Ge⁺ ions. At the next step energy at a level between 1100 eV and1200 eV is applied to accelerate the Ge⁺ ions to a substrate. Then, thesubstrate is heated in the range of 200° C.-380° C., thereby forming anepitaxial of CoGe₂ layer to serve as an electrical contact with the GaAsdoped substrate.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective diagram depicting a device for using the processof the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The process of the present invention forms an epitaxial layer of acobalt germanide, CoGe₂ on GaAs to form the heteropitaxial system ofCoGe₂, (001)/GaAs (100). This phase offers a low lattice mismatch(-0.2%) with GaAs, and has been shown in bulk form to be the minimumresistivity phase of all the cobalt germanides with a polycrystallineresistivity of 3.5 μΩ-cm., a value comparable to that of CoSi₂, and hasa high melting temperature (806° C.). Therefore, single-crystal contactsof this phase can provide excellent high-temperature thermal stability.

The inventive process is carried out by modifying the conventionalpartially ionized beam (PIB) deposition operation as depicted in FIG. 1.The PIB process facilitates use with the present invention because ofcertain characteristics of the PIB process described below.

Conventionally, in the formation of heteropitaxial layers, one muststart with a clean surface free of native oxides and contaminants, sothat the arriving evaporant atoms "see" clearly the periodic potentialof the substrate. To achieve such a perfect surface, in-situ cleaning isnecessary. In-situ cleaning typically consists of either sputteringclean the surface with Ar or Xe ions of several kV, and subsequentlyannealing out the surface damage, or heating the substrate to the pointwhere impurities are desorbed. With Si these techniques work well.However, GaAs, being a binary compound, suffers from preferentialsputtering of surface atoms and differing vapor pressures of Ga and As.Therefore, both techniques lead to a non-nonstoichiometric surface. Toachieve a perfect surface with GaAs, one must have an As overpressurewhile heating to desorb impurities, or a new layer of one 1:1 GaAs isgrown in-situ after the substrate has been cleaned. Heteroepitaxialfilms must also be grown under high vacuum conditions, as the surface isquickly recontaminated if the pressure is greater that about 10⁻⁹ Torr.

The situation for PIB deposition is quite different. In this case, asmall fraction of the evaporated species is ionized, typically <1%. Bythe application of a bias to the substrate one may obtain ionbombardment of first the semiconductor surface, and then the growthfront of the growing film. One of the principal advantages of the PIBtechnology is the capability of depositing heteroepitaxial films atlower (than conventional) substrate temperatures in a conventionalvacuum without in-situ cleaning of the substrate prior to deposition. Ineffect the PIB technique possesses a surface self-cleaning capability.

Also, for microelectronics applications, deriving the ions from thesource material is preferred, since self-ions do not contaminate thefilm. Ions in the evaporant stream increase surface mobility, provideaddition energy to the growth front, sputter away light impurities, andcan control nucleation characteristics. The PIB deposition methodexhibits the aforementioned characteristics, and thus, lends itself touse with the novel parameters of the present invention.

The PIB deposition system as depicted in FIG. 1 is used to deposit thealloy films on GaAs (100) substrates 14. The PIB system is equipped witha graphite crucible 10 surrounded by a Ta filament 12. Electrons instream 21 are thermionically emitted from the powered Ta filament, andstrike the graphite crucible 10 due to the application of a highpositive voltage, typically ≈1200V. A portion of the electrons travelover the mouth of the crucible and impact ionize the exiting Ge vaporstream 20 (less than 1% and approximately 0.2%). These Ge⁺ ions are thenaccelerated by a potential from source 13 applied to the substrate 14.The Ge⁺ ions arrive at the growth front of substrate 14 with an energygiven by the potential difference between the crucible and thesubstrate, and are deposited along with the neutral Ge species (fromstream 20) and Co, (from stream 23) which is conventionally evaporatedin a resistively, heated BN crucible 15, powered by electrical source27. The deposition rates were selected such that the ratio of the Co toGe atoms arriving at the substrate is 1:2.

Substrate preparation consists of a 30 second wet etch in HF:H₂ O 1:10,followed by drying in flowing N₂ gas. However, other substratepreparations can be used with the present invention. Immediately afterthe etch, the wafers were loaded into a processing chamber (not shown)for deposition. The interior of the chamber containing the substrate andother necessary elements for the deposition process (including shutters25 crystal monitor 26 and heater 18) are also depicted in FIG. 1.

In the chamber a planar W heating filament 18 is positionedapproximately 3 cm above the substrate 14 to heat the sample duringdeposition. The GaAs substrate is positioned under the W filament, wasallowed 3 minutes to reach the correct temperature (optimally 280° C.)before starting an alloy deposition. The depositions proceeds under avacuum of ≈5×10⁻⁷ Torr, and the films are preferably grown to athickness of 1500 Å. The system base pressure was 10⁻⁷ Torr.

The deposition is preferably carried out on n-type epi-ready GaAs (100)wafers. For electrical measurements, 4.42×10⁻³ cm² dots are depositedthrough a shadow mask (not shown) onto Si doped GaAs (100) wafers with acarrier concentration of (1-4)×10¹⁸ cm⁻³.

The Ge⁺ ions arrive at substrate 14 with an energy given by thepotential drop between the crucible 10 and the substrate 14 and aredeposited along with the neutral Ge species and the Co. In general thepresent invention can be practiced to grow epitaxial layers at an energylevel range of ˜1100 eV to 1200 eV. Since Co undergoes a magnetictransition at ˜1100° C. it was found to be incompatible with PIBevaporation. Consequently, the Co must be heated in a resistively heatedcrucible 15. The deposition rates are preferably selected such that theratio of the Co and Ge atoms arriving at the substrate was 1:2. Thedeposition rates for the Co and Ge are ˜0.5 and 2.1 Å/s. respectively.

The temperature range of the heated substrate 14 varies from 200° C. to380° C. for the inventive process. Analysis of the phase of the depositshowed that the CoGe₂ phase formed most readily at a substratetemperature of approximately 280° C. Consequently, this is one of theoptimal parameters of the present invention. However, the presentinvention can be facilitated at other, less optimal temperatures in theaforementioned range. At very low substrate temperatures (≦200° C.) onlya small degree of crystallinity in the films were observed. In addition,at a substrate temperature of 390° C., the film was also found topossess little crystallinity.

The films deposited under ideal conditions of ˜280° with 1100 eV Ge⁺ions display a very tight epitaxial structure with the substrate 14.Stereograms taken of the films and substrates formed by the inventiveprocess are able to detect 4 GaAs (111) poles and 4 CoGe₂ (111) poles.The orientation and the angle of the poles for both the epitaxial layerand the substrate were found to coincide, thereby creating an extremelystable lattice configuration between the substrate and the epitaxiallayer. The resulting product is a parallel epitaxial arrangement ofCoGe₂ (001) /GaAs (100).

Current-voltage measurements were preformed on 4.42×10⁻³ cm² contactsgrown under the aforementioned conditions for epitaxial growth and thestructure was found to exhibit Ohmic behavior. Contacts grown atsubstrate temperatures above or below the ideal substrate temperaturerange of approximately 260°-380° C. were found to be rectifying.

It has been discovered that films deposited at 280° C. with lower ionsenergies display a different, nonparallel epitaxial arrangement thanthat of the structure formed under the aforementioned optimumconditions. This same deficiencies are also found for those filmsdeposited with a lower substrate temperature of 200° C. and an ionenergy near 1100 eV. Lowering the substrate temperature or reducing ionenergy both seem to produce a deteriorated final product. Apparently thestream of Ge⁺ ions 22 formed in the evaporant stream (22,23,24) increasethe surface mobility, provide additional energy to the growth front,sputter away light impurities and also suppress three dimensional islandgrowth by increasing the density of nucleation sites thus facilitatingthe high epitaxial growth that is the chief advantage of the presentinvention.

Atomic force microscopy (AFM) analysis of the products manufacturedaccording to the inventive process shows that the surface of theepitaxially grown films are rough, with a root mean square roughness of400 Å for a 1500-Å-thick film. It is possible that preferential facetingis responsible for the roughness. However, the film is found to becontinuous with no observable grain boundaries. Based upon the highintensity of the CoGe₂ (004) and (008) peaks in a 2θ scan of the finalproduct and the fact that the (111) poles in the stereogram areremarkably tight (as narrow as the substrate poles), the resultingepitaxial film is found to be almost completely a single crystallinestructure.

Using the PIB system as modified by the parameters of the presentinvention, epitaxial CoGe₂ is a viable alternative to the metallizationof n-GaAs. Although a number of preferred embodiment of this inventionhas been mentioned by way of example, it is not intended that theinvention be limited thereto. Accordingly, the invention should beconsidered to include any and all configurations, modifications,variations, combinations, equivalent arrangements or expansions fallingwithin the scope of the following claims.

What is claimed:
 1. A process for forming CoGe₂ electrical contacts on aGaAs doped substrate (14), comprising the steps of:(a) generating apartially ionized beam (20, 22, 23) including Ge⁺ ions (22); (b)applying energy in a range of 1100 eV through 1200 eV to said Ge⁺ ions;thereby accelerating said partially ionized beam to a GaAs dopedsubstrate upon which electrical contacts are to be formed; (c)generating a stream of Co and directing the stream to the GaAs dopedsubstrate; (d) heating said substrate to a temperature between 200° C.through 380° C.; (e) simultaneously to step (d) creating a vacuum aroundsaid substrate to a level of approximately 5×10⁻⁷ Torr.
 2. The processof claim 1, wherein step (c) is carried out by heating said substrate tosubstantially 280° C.
 3. The process of claim 2, wherein step (c) iscarried out for a duration of approximately 3 minutes.
 4. The process ofclaim 1, wherein step (b) is carried out by applying substantially 1100eV to said Ge⁺ ions (22).
 5. The process of claim 1, wherein step (a)comprises the substep of:(i) evaporating Ge (20).
 6. The process ofclaim 5, wherein substep (i) is carried out in a graphite crucible. 7.The process of claim 5, wherein step (a) further comprises a substepof:(ii) generating electrons (21) by thermionic emission.
 8. The processof claim 7, wherein said electrons(21) are emitted by heating a Tafilament (18) above the thermionic emission temperature.
 9. The processof claim 8, wherein said Ge⁺ ions (22) are generated by impacting saidelectrons against said evaporated Ge (20).
 10. The process of claim 7,wherein substep (iii) is carried out in a resistively heated crucible(16).
 11. The process of claim 1, further comprising the preliminarystep of washing said substrate in a solution of HF:H₂ O at a 1:10 ratioprior to step (a).
 12. The process of claim 1, wherein said partiallyionized beam (20,22,23) includes Co and Ge in a ratio of 1:2.
 13. Theprocess of claim 11, wherein prior to said step of washing saidsubstrate is implanted with Si at a level in the range of (1-4)×10¹⁸cm³.
 14. A process for forming CoGe₂ electrical contacts on a GaAs dopedsubstrate comprising the steps of:(a) generating a partially ionizedbeam including Ge⁺ ions; (b) generating a stream of cobalt; directingsaid ionized beam and said stream of cobalt to a GaAs doped substrateunder conditions such that said Ge⁺ ions and said stream of cobalt formCoGe₂ electrical contacts on the substrate.
 15. The process of claim 14wherein the substrate is a silicon doped GaAs substrate.